1. Technical Field
The present invention relates to an output driving circuit and a transistor output circuit, and more particularly, to an output driving circuit and a transistor output circuit that stably operate when a high voltage that is smaller than a source-drain breakdown voltage and is greater than a gate-source breakdown voltage is applied to a gate of an output transistor.
2. Description of the Related Art
An operating voltage of a P-channel transistor, for example a P-channel LDMOS is determined according to a source-drain breakdown voltage BVsd, a source-gate breakdown voltage BVsg, and a gate-drain breakdown voltage BVgd. From among these breakdown voltages, the source-gate breakdown voltage BVsg is the lowest. The source-gate breakdown voltage BVsg is determined according to a thickness of a gate oxide. This is because the gate oxide may not be formed to have an excessive large thickness since a threshold voltage Vth of a transistor, a source-drain current Isd, on-resistance Ron, and the like are determined according to a thickness of an oxide. If a power voltage is lower than the source-drain breakdown voltage BVsd and is higher than the source-gate breakdown voltage BVsg, when the power voltage is applied to a source terminal and a low voltage (ground, etc.) is connected to a drain terminal, no problem arises. However, if the power voltage and the low voltage (ground) are applied to a gate terminal in order to operate a transistor, the power voltage is applied to a source-gate voltage Vsg. In this case, since the power voltage is greater than the source-gate breakdown voltage BVsg, the transistor device may breakdown.
FIG. 5 is a schematic circuit diagram of a conventional output driving circuit.
Referring to FIG. 5, when a switch SW1 is switched off in order to turn on an output transistor T1, if a current I flows through a power voltage VDD, a resistor R connected to a gate of the output transistor T1, and a Zener diode Z1, the output transistor T1 is driven by lowering a source-gate voltage of the output transistor T1 compared to the source-gate breakdown voltage BVsg and increasing the source-gate voltage compared to a threshold voltage Vth by using the Zener diode Z1. On the other hand, the output transistor T1 is turned off by switching on the switch SW1 and increasing a gate voltage of the output transistor T1 up to the power voltage VDD by using the resistor R connected to the power voltage VDD and the gate of the output transistor T1.
In order to operate the output transistor T1 at a relatively high frequency, a current mirror instead of the resistor R may be used to turn off the output transistor T1, unlike in FIG. 5.